The invention relates to an ion sensor which can be used, for example, for determining the energy spectrum of positively charged ions in a plasma. Such an energy analysis is required, for example, in the testing of ion thrusters in order to be able to obtain information about the efficiency of the thruster.
The ion sensor under consideration here comprises as core components an electron repelling electrode and an ion repelling electrode. The electron repelling electrode is placed at a negative electric potential during operation of the ion sensor, whereby there is formed an electrostatic field which decelerates and finally repels the electrons contained in an inflowing plasma, so that substantially only the positively charged ions contained in the plasma are able to pass through the negative electrostatic field. The ion repelling electrode is, for its part, placed at a positive electric potential, this positive electric potential generally being variable in terms of its strength. Depending on the strength of the positive electrostatic field, ions up to a specific energy are repelled by the field; only ions of sufficiently high energy pass through the barrier formed by the positive electrostatic field. Finally, these ions come into contact with a metallic collector face, where they cause a current flow in the metal material forming the collector face. This electric current can be measured; information about the energy distribution of the ions contained in the plasma can be obtained from the gradient of the current flow, which changes in dependence on the positive potential that is applied.
Ion sensors of this type are referred to in the art as retarding potential analyzers, analyzers which operate with a retarding potential by which first the electrons and then the positively charged ions—in dependence on their energy—are decelerated. In accordance with their barrier function, the electron repelling electrode and the ion repelling electrode are frequently referred to as repellers in the art.
In a conventional type of RPA ion sensor, the repeller electrodes are formed by grids. Another conventional type of RPA ion sensor does not employ grid electrodes. Instead, it uses annular electrodes which, in contrast to a grid, provide only a single through-opening for the plasma components and are therefore also referred to in the art as single orifice RPAs. For a known form of such a single orifice RPA, reference is made to the article “High Precision Beam Diagnostics for Ion Thruster nu s” by Benjamin van Reijen et al., published in “The 32nd International Electric Propulsion Conference”, 11-15 Sep. 2011, Wiesbaden, Germany, IEPC-2011-132. The RPA ion sensor described therein has an annular-cylindrical ion repelling electrode behind which there is arranged, at a distance therefrom, a sensor electrode serving as the ion collector. Between the ion repelling electrode and the sensor electrode there is arranged an additional auxiliary electrode, which is to be placed at a negative electric potential and serves on the one hand to focus the ion beam passing through the ion repelling electrode and on the other hand as a repeller for any secondary electrons which may form when the ions come into contact with the sensor electrode.